Trying to use MassMessage yields the error above. I've tried with several test messages and target lists.
2016-01-22 13:14:39 mw1257 metawiki exception ERROR: [079ab5da] /wiki/Special:MassMessage MWException from line 3662 of /srv/mediawiki/php-1.27.0-wmf.11/includes/user/User.php: CAS update failed on user_touched for user ID '3580349' (read from slave); the version of the user to be saved is older than the current version. {"exception_id":"079ab5da"} [Exception MWException] (/srv/mediawiki/php-1.27.0-wmf.11/includes/user/User.php:3662) CAS update failed on user_touched for user ID '3580349' (read from slave); the version of the user to be saved is older than the current version. #0 /srv/mediawiki/php-1.27.0-wmf.11/includes/session/SessionManager.php(534): User->saveSettings() #1 /srv/mediawiki/php-1.27.0-wmf.11/includes/user/User.php(679): MediaWiki\Session\SessionManager->preventSessionsForUser(string) #2 /srv/mediawiki/php-1.27.0-wmf.11/extensions/MassMessage/includes/MassMessage.php(44): User::newSystemUser(string, array) #3 /srv/mediawiki/php-1.27.0-wmf.11/extensions/MassMessage/includes/SpecialMassMessage.php(291): MassMessage::getMessengerUser() #4 /srv/mediawiki/php-1.27.0-wmf.11/extensions/MassMessage/includes/SpecialMassMessage.php(189): SpecialMassMessage->preview(array) #5 /srv/mediawiki/php-1.27.0-wmf.11/includes/htmlform/HTMLForm.php(607): SpecialMassMessage->callback(array) #6 /srv/mediawiki/php-1.27.0-wmf.11/includes/htmlform/HTMLForm.php(504): HTMLForm->trySubmit() #7 /srv/mediawiki/php-1.27.0-wmf.11/extensions/MassMessage/includes/SpecialMassMessage.php(71): HTMLForm->tryAuthorizedSubmit() #8 /srv/mediawiki/php-1.27.0-wmf.11/includes/specialpage/SpecialPage.php(384): SpecialMassMessage->execute(NULL) #9 /srv/mediawiki/php-1.27.0-wmf.11/includes/specialpage/SpecialPageFactory.php(564): SpecialPage->run(NULL) #10 /srv/mediawiki/php-1.27.0-wmf.11/includes/MediaWiki.php(280): SpecialPageFactory::executePath(Title, RequestContext) #11 /srv/mediawiki/php-1.27.0-wmf.11/includes/MediaWiki.php(738): MediaWiki->performRequest() #12 /srv/mediawiki/php-1.27.0-wmf.11/includes/MediaWiki.php(510): MediaWiki->main() #13 /srv/mediawiki/php-1.27.0-wmf.11/index.php(43): MediaWiki->run() #14 /srv/mediawiki/w/index.php(3): include(string) #15 {main}